Liners are frequently attached to casing using hydraulically set slips and external casing packers. In order to actuate these hydraulically activated components, the liner string is provided with a landing collar which consists of a seat which accepts a sphere for obstruction of the central passage. Pressure is thereafter built up to actuate the hydraulic components to suspend the liner to the casing and/or to actuate packers. Typically, when the liner is secured, the passage must be reopened to allow cement to be pumped therethrough. At the conclusion of the cementing, the landing collar could be drilled out to reopen full-bore capabilities in the liner.
In situations where the formation is sensitive, the procedure for reestablishing flow in the liner created shocks of pressure into the formation. The reason this occurred is that the sphere landed on the seat would experience a pressure build-up beyond a predetermined value until a shear pin or pins would break. Generally, the ball and seat would move in tandem after the shear pin broke and such movement would instantaneously open a passage to the formation below. Thus, the built-up pressure behind the ball seated on the seat would very quickly create a pressure shockwave into the formation. The pressure to shear the pins was typically several thousand pounds per square inch. A large volume of fluid is generally present above the ball. This large volume contains a large amount of stored energy from the compressibility of the fluid itself and any dissolved gases that are in it. In addition, the applied pressure flexes the tubing above the ball which, upon relief of pressure, adds to the force behind the shockwave on the formation. The hydraulic shock to the formation is undesirable because it can cause damage to sensitive formations which can result in formation breakdown or severe fluid losses.
Prior designs which have retained the landing collar with shear screws have generally employed brass or bronze shear screws inserted into aluminum components. During applications involving elevated temperatures, such as above 350.degree. F., the aluminum softens and the breakpoint of shear screws experiences a decline in reliability so that the breakpoint can be plus or minus 15% of the expected value. The use of harder metals in this type of a structure is undesirable because occasions can arise where the landing collar needs to be drilled out for subsequent downhole operations.
The tubular structure which comprises the seat has, in previous designs, been spring-loaded and secured to the housing in a pin-and-slot arrangement so that a succession of applications and removals of pressure could be used to advance the pin in the slot until eventually, the pin reached an open portion of the slot. When so aligned, the assembly of the seat and sphere would simply fall down the liner or be caught slightly below its initial position with only a minimal applied pressure. This type of structure was generally made of hard steels to facilitate its reliable operation. However, one of the problems that ensued with such a design, if it had to be drilled out, is that it took a long time to do that because of the hardness of the various components. This design could also jam due to the numerous movements required to release it.
Accordingly, what was needed and is necessarily an object of the present invention is a design which is simple and yet reliable. The objective of the present invention is to reduce, if not eliminate, shocks to the formation resulting from displacement of the ball-and-seat combination after the actuation of the hydraulic components downhole. Another objective accomplished by the simplicity of the design is to facilitate the use of softer materials, such as nonmetallic components so that subsequent drilling out, if necessary, can be accomplished quickly. Yet another objective is to provide greater reliability of actuation at a predetermined pressure level. This is in part accomplished by moving away from shear pin designs for normal operation to alternatives which have a demonstrated closer tolerance to actuation at a predetermined pressure. Those and other objectives will be more readily understood by a review of the preferred embodiment of the invention as described below.